The invention relates to a protocol (communication control procedure) in LANs (Local Area Networks), and more particularly, to a communication system for controlling communications by detecting contentions in a network.
Generally, in a LAN a plurality of nodes are connected to a bus and the nodes are intercommunicated through the bus. A multiple access bus is one of the buses used in such a LAN. The term "multiple access bus" means a bus that allows a signal transmitted from a certain node to be received by all the nodes simultaneously. Ethernet.RTM. is one well known LAN using this multiple access bus and its protocol is called "CSMA/CD (Carrier Sense Multiple Access/Contention Detection) and standardized as IEEE 802.3.
Ethernet.RTM. employs a coaxial cable as a transmission medium. Nodes are connected to the coaxial cable and a transmitting node first checks whether a signal is present on the coaxial cable from another node and then starts its transmission if such signal is absent. However, in such a case, it is likely that two or more nodes will start transmitting messages simultaneously. This condition is called a "contention" and Ethernet.RTM. detects contentions by a voltage level.
A node that has detected a contention, waits for a random time after sending a jamming signal for a predetermined period of time. This jamming signal must be longer than the maximum round trip time within the network. The jamming signal serves to inform all the nodes connected to the network of the contention. The reason why the node waits for a random time is that, if the nodes that failed to transmit a message try to make transmissions all together simultaneously when the bus is not busy, a contention occurs again. Thus, each node is made to wait randomly to avoid recurrence of contentions.
Meanwhile, there is an increasing tendency to apply optical communication systems to LANs. However, optical LANs using optical fibers as transmission media cannot be expanded simply by providing taps as in the case of LANs configured by using a coaxial cable, to add nodes to the networks.
To overcome this problem, a network configuration has been proposed, in which terminals are provided separately for transmission and reception per node and star couplers are employed to distribute a signal to all nodes (E.G. Rawson, "Fibernet: Multimode Optical Fibers for Local Computer Networks," IEEE Transactions on Communications, Vol. COM-26, No. 7, July 1978).
If this star coupler is of a passive type, an increase in the number of nodes to be connected to such a star coupler leads to reduce the level of a received signal per node. To avoid this problem, it is conceivable to use a star coupler of an active type. However, a new problem arises therefrom when nodes are to be additionally provided. That is, when accommodating the addition of nodes by combining a plurality of star couplers, each star coupler having a predetermined number of terminals, a feedback loop is formed between the interconnected star couplers, causing inconvenience such as oscillation. For this reason, when using an active star coupler, the maximum number of connectable nodes is equal to the number of terminals predetermined per star coupler.
To overcome this problem, the applicant of the invention proposed in Published Unexamined Japanese Patent Application No. 296332/1991 a technique for forming no feedback loop even if a plurality of star couplers are combined. The technique involves the star coupler in which every transfer constant between input terminal and output terminal is zero. And in an optical communication network configured by interconnecting such star couplers as shown in the specification of the above application, a transmitting node does not have its transmitting signal returned to itself. Therefore, a node in transmission is ready to receive a signal from another node, allowing the node to transmit and receive signals simultaneously. That is, the optical communication network disclosed in the aforesaid specification forms a two-way bus.
With respect to a network configured by a combination of star couplers, the present inventor proposes two-way communication implemented by a single optical fiber in the co-pending U.S. patent application no. 07/813,443 the disclosure of which is incorporated herein for reference.
To implement two-way communication on the aforesaid two-way bus, when communication is started between a first node and a second node, the possibility is not excluded that a third node, i.e., a node other than the concerned party, will start transmitting a message. The contention caused by the third node is similar to a contention caused by another transmitting node in one-way communication.
In a network using a multiple access bus such as in Ethernet.RTM., a signal transmitted by a node can be received by all other nodes, which is not preferable from the viewpoint of confidentiality.